build-sys: Add setns() functions if not in the C library.

Debian Stable is still using glibc 2.13, which doesn't provide the setns().
So we detect this and provide a tiny wrapper that issues the setns syscall
towards the kernel.
missing: simplifications
build-sys: Fix compilation of nspawn when seccomp is not enabled
build-sys: don't use -Wcast-align anymore

We do a lot of these casts, and they are all OK, so let's get rid of the
warning, to make things quieter.

http://lists.freedesktop.org/archives/systemd-devel/2014-February/017174.html